The present invention finds particular utility in a class of electromagnetic relays called plug-in relays wherein external connections can be made by plugging the relay into a receptacle mounted on a panel board or racks and can be broken by unplugging it therefrom. Further, when the relays are mounted on the supporting racks they are relatively close together so that there is very little space between adjacent relays as they project forward from the panel. Generally, these plug-in relays are manufactured in protective casings which include a front cover member and a back relay base member. The cover member and base member enclose the electrical and magnetic components and protect them from the adverse conditions of the environment. In addition to dirt, dust, moisture and other foreign matter, a railroad environment is susceptible to extreme and excessive vibrations which tend to loosen joined parts and components. Thus, it is advisable to employ a locking or latching mechanism with plug-in relays to prevent the male terminal base from becoming disconnected from the female mounting sub-base which is carried by the supporting panel. While previous latching mechanism have been effectively utilized in plug-in relays for railroad installations, these latches were expensive to manufacture and assembled, were relatively bulky and used up premium space and were unduely complicated in construction and operation. Thus, it is desirable to provide an improved latching device which is relatively inexpensive and simple in construction. Further, it is highly advantageous in relays of reduced size to minimize the space requirements and the complexity of the latching mechanism.